linux-atm: Fix compile problem with kernel 4.20

This fixes the following compile problem with kernel 4.20:

In file included from arp.c:20:0:
include/linux/if_arp.h:121:16: error: 'IFNAMSIZ' undeclared here (not in a function)
  char  arp_dev[IFNAMSIZ];
                ^~~~~~~~
make[7]: *** [Makefile:459: arp.o] Error 1

This is caused by commit 6a12709da354 ("net: if_arp: use define instead
of hard-coded value") in the upstream Linux kernel which is integrated
in Linux 4.20.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

package/network/utils/linux-atm/patches/510-remove-LINUX_NETDEVICE-hack.patch

@@ -0,0 +1,52 @@
+This fixes the following compile problem with kernel 4.20:
+
+In file included from arp.c:20:0:
+include/linux/if_arp.h:121:16: error: 'IFNAMSIZ' undeclared here (not in a function)
+  char  arp_dev[IFNAMSIZ];
+                ^~~~~~~~
+make[7]: *** [Makefile:459: arp.o] Error 1
+
+This is caused by commit 6a12709da354 ("net: if_arp: use define instead 
+of hard-coded value") in the upstream Linux kernel which is integrated 
+in Linux 4.20.
+
+--- a/src/oamd/io.c
++++ b/src/oamd/io.c
+@@ -20,7 +20,6 @@
+ #include <net/if.h>
+ #include <netinet/in.h>
+ #include <atm.h>
+-#define _LINUX_NETDEVICE_H	/* glibc2 */
+ #include <linux/types.h>
+ #include <linux/if_arp.h>
+ 
+--- a/src/arpd/itf.c
++++ b/src/arpd/itf.c
+@@ -12,7 +12,6 @@
+ #include <sys/types.h>
+ #include <linux/atmclip.h>
+ #include <sys/socket.h>
+-#define _LINUX_NETDEVICE_H /* glibc2 */
+ #include <linux/types.h>
+ #include <linux/if_arp.h>
+ 
+--- a/src/arpd/io.c
++++ b/src/arpd/io.c
+@@ -21,7 +21,6 @@
+ #include <atm.h>
+ #include <linux/atmclip.h> /* for CLIP_DEFAULT_IDLETIMER */
+ #include <linux/atmarp.h>
+-#define _LINUX_NETDEVICE_H /* glibc2 */
+ #include <linux/types.h>
+ #include <linux/if_arp.h>
+ 
+--- a/src/arpd/arp.c
++++ b/src/arpd/arp.c
+@@ -15,7 +15,6 @@
+ #include <sys/types.h>
+ #include <sys/socket.h> /* for linux/if_arp.h */
+ #include <netinet/in.h> /* for ntohs, etc. */
+-#define _LINUX_NETDEVICE_H /* very crude hack for glibc2 */
+ #include <linux/types.h>
+ #include <linux/if_arp.h>
+ #include <linux/if_ether.h>